DESCRIPTION: Stroke is a major health care problem because is has a high incidence, its treatment is expensive, it involves a substantial use of health care resources, and patients face a potentially poor quality of life. A significant risk of stroke is present in certain subgroups of patients, such as those with carotid occlusion and contralateral high-grade stenosis who have had a transient ischemic attack (TIA) or previous stroke on the side of occlusion. When a high-grade carotid stenosis is found with contralateral carotid occlusion, patients most often undergo carotid endarterectomy (CEA) on the side of the stenosis. Nevertheless, CEA will most likely be beneficial (i.e., increase flow or eliminate an embolic source) only on the side of the surgically corrected stenosis. Thus, the risk of stroke will remain in some patients with bilateral disease of the internal carotid artery. Such patients represent a significant portion of patients with symptoms contralateral to the CEA. A reliable and highly predictive method is needed to identify those who would or would not benefit from CEA. It is hypothesized that the measurement of cerebrovascular reserves will enable the evaluation of benefit to the occluded hemisphere afforded by treatment of contralateral carotid stenosis. The plans are to use stable xenon-enhanced computed tomography (XeCT) and/or transcranial Doppler (TCD) ultrasonography with an acetazolamide-induced blood flow challenge to examine preoperative and postoperative cerebrovascular reserves. The patients will also be followed up for at least 2 years to determine their postoperative course. The purpose of the proposed study is to assess the potential role of cerebrovascular hemodynamic measurements in deciding how to treat patients with symptomatic carotid occlusion and contralateral high-grade stenosis.